Electromagnetic flow meters are used in a variety of industries to monitor the flow of conducting fluids. Electromagnetic flow meters utilise Faraday's law of electromagnetic induction to induce a voltage in the conducting fluid as it moves through a magnetic field. The flow rate of the conducting fluid is then derived from the measured induced voltage.
In a conventional electromagnetic flow meter the conducting fluid is directed to flow through a flow pipe, electromagnetic coils are located outside the flow pipe to create a magnetic field, two electrodes are mounted in the flow pipe wall to detect the induced voltage and processing means are configured to process the induced voltage data to determine the average flow rate. Although conventional electromagnetic flow meters are widely used it is recognised that they have a number of limitations. For example, conventional electromagnetic flow meters can only measure the average flow rate of a conducting fluid—they can not determine the axial velocity profile of a conducting fluid. Moreover, conventional electromagnetic flow meters are generally only effective when the conducting fluid has a uniform flow profile—they are unsuitable and/or inaccurate when the conducting fluid has a non-uniform flow profile.
Unfortunately, non-uniform flow conditions are often encountered. For example, a fluid may develop a non-uniform flow profile downstream of a pipe bend, at a partially open valve, in a blocked pipe and/or along an inclined pipe. A multiphase fluid may have a non-uniform flow profile if the component parts have different flow characteristics. The flow of a fluid is non-uniform when, for example, the fluid has a non-axisymmetric velocity profile.
One approach to accurate flow rate measurement of non-uniform single phase fluids has been proposed by HORNER in HORNER, B. (1998) A novel profile-insensitive multi-electrode induction flow meter suitable for industrial use. Meas. Sic. Technol., 24, 131-137. However, this type of flow meter does not provide information on the axial velocity profile of the fluid. This can be a major drawback, particularly in multiphase fluids where, for example, the volumetric flow rate of a particular phase can only be found by integrating the product of the local phase velocity and the local phase volume fraction in the flow cross section. As a result, the approach proposed by Horner can not be used to determine the flow rate of the conducting continuous phase of a multiphase fluid with a non-uniform flow profile.
Other known types of flow meters that are suitable for measuring the flow rate of a conducting phase of a multiphase fluid are constrained by high cost and the use of hazardous radioactive sources to monitor the flow.